1. Field of the Invention
The present invention relates to a method and apparatus for gamma correction and white balance adjustment in a projection display device wherein color images are obtained by synthesizing red, blue, and green projection images.
2. Description of the Prior Art
A typical projection display obtains a color image by projecting and synthesizing on screen three images each of a single color, specifically red, green and blue. Projection display systems can be either a self-luminescent system (e.g., CRT system) or light valve system (e.g., liquid crystals used as light valves) depending on the method of generating the projection image.
In the self-luminescent system a CRT or other light-emitting device is used to generate three primary color (red, green, blue) images which are projected by an optical projection system to overlap on the screen and form a full color image. The CRT projection display system uses three CRTs to generate the separate red, green, and blue images. A widely used method for adjusting the white balance in the self-luminance system is to adjust the RGB source signal levels in each CRT and to absorb individual differences in the emission intensity and spectrum of each CRT.
In the light valve system, three primary color (red, green, blue) images are projected by an optical projection system to overlap on screen and form a full color image by passing light from red, green, and blue backlight sources through corresponding light valves, and controlling the transmittance of the passed lights at each of the light valves.
Recent broadcasting systems apply gamma correction for CRT (referred to as a CRT gamma correction) at the broadcasting station so that the transmitted signals already contain CRT gamma correction characteristics. However, because every CRT projection system including its driving circuit has its own characteristics, the CRT gamma correction added to the broadcasting signal does not always match the CRT gamma correction required in each CRT projection system. Thus, it is necessary to apply CRT gamma correction to each CRT projection system in order to achieve gradation characteristics.
In the light valve system and in non-CRT self-luminescent systems, the gradation characteristics of the image are dependent upon the emission characteristics of the light emitting element or the transmission characteristics of the light valves.
The transmission characteristics of the light valve in a liquid crystal light valve system is shown in FIG. 8a. The light valve also has its own characteristics, the gamma correction for the light valve (referred to as a LV gamma correction) is required to each and individual light valve projection display system to obtain gradation characteristics equivalent to a CRT display. Various circuit designs have been proposed to obtain LV gamma correction characteristics, and one example is shown in FIG. 9a.
This circuit comprises a first circuit 110 for compressing a preset video signal component of &gt;3 V, and a second circuit 111 for limiting the compression operation of the first circuit 110. The LV gamma correction characteristics shown in FIG. 9b is obtained by the second circuit 111 functioning to restrict the compression operation of the first circuit 110 when the video signal component compressed by the first circuit 110 exceeds a preset voltage (4 V). The result is an input/output characteristic that approximates the LV gamma correction curve (FIG. 9b) for the transmittance-voltage characteristics of the liquid crystals.
The conventional method can provide basic white balance and gradation characteristics control. In a conventional direct-view CRT television, however, the white balance is dependent upon the emission strength and emission spectrum of the RGB phosphors in the CRT surface, and the RGB color signal levels. In a CRT projection display system, however, the image white balance is also dependent upon the spectral characteristics of the RGB image projection optics. In the light valve projection display system, the white balance depends upon the light valves, the transmittance of the light valves to each color, and the spectral characteristics of the projection optics. As a result, conventional methods of applying level adjustment and gamma correction to each signal are unable to provide precision white balance tracking across all RGB color signal levels.